In recent years, development and commodification of a fuel cell system as a distributed power generation system progresses. The fuel cell system is supplied as a raw material an organic compound containing carbon and hydrogen components. The fuel cell system generates hydrogen by reforming the supplied raw material, for example, within a fuel cell. Or, the fuel cell system includes a reformer outside the fuel cell, and the reformer reforms the raw material to generate a reformed gas containing hydrogen. The fuel cell generates electricity and heat, through a power generation reaction by using hydrogen thus generated and air supplied from outside.
The fuel cell system configured as described above can efficiently attain electric energy and heat energy, and therefore is expected as an energy supply system which can effectively reduce carbon dioxide which is a cause of global warming.
As the raw material supplied to the fuel cell system, for example, a liquefied petroleum gas (LPG), a liquefied natural gas (LNG), a city gas, a shale gas, methane hydrate, etc., may be used. Each of these raw materials contains an odor substance added thereto. The raw material itself or the odor substance added to the raw material contains a sulfur component. If the raw material containing this sulfur component is supplied to an anode of the fuel cell through the reformer, or the like, this sulfur component may poison the anode, which will result in a situation in which fuel cell performance is degraded, or the reforming catalyst included in the reformer is poisoned and thereby a reforming capability is degraded. Because of this, it becomes necessary to supply the raw material to the anode and the reformer after the sulfur component in the raw material is reduced to ppb order.
To this end, the fuel cell system includes a desulfurization unit having a function of reducing the sulfur component in the raw material, at the upstream side of the reformer. As a method of removing the sulfur component from the raw material by the desulfurization unit, there is a room temperature desulfurization method that removes the sulfur component by physically adsorbing the sulfur component to a catalyst at a room temperature, a hydrodesulfurization method that removes the sulfur component by using hydrogen, etc. In the case of the hydrodesulfurization method, the catalyst having an active temperature range of a predetermined temperature (e.g., about 250 degrees C. to 320 degrees C.) is filled into the desulfurization unit. The desulfurization unit generates hydrogen sulfide from the raw material and hydrogen supplied from outside, and chemically adsorbs sulfur of hydrogen sulfide to the catalyst.
By the way, it is necessary to heat the desulfurization unit to keep the desulfurization unit at the predetermined temperature (e.g., about 250 degrees C. to 320 degrees C., in the case of the hydrodesulfurization method), to steadily remove the sulfur component. For example, Patent Literatures 1, 2 disclose fuel cell systems which are configured to heat the desulfurization unit up to the predetermined temperature.
Specifically, Patent Literature 1 discloses the fuel cell system which is configured to heat the desulfurization unit in the manner as described below. An anode off-gas discharged from the anode of the fuel cell and a cathode off-gas discharged from the cathode of the fuel cell are combusted together to generate a combustion exhaust gas (fuel cell exhaust gas). This combustion exhaust gas is caused to exchange heat with cathode air to be supplied to the cathode, and a part of the heated cathode air is supplied to the desulfurization unit as a heat source.
Patent Literature 2 discloses the fuel cell system in which water (reforming water) supplied to the reformer as a reforming material deprives heat from a burner, and this heat is transferred to a desulfurization catalyst, to keep the desulfurization unit at a predetermined temperature.